CODEWITHSUNDEEP
for-loopforeachtypescript
Mick
Mick

Reputation: 8913

TypeScript for ... of with index / key?

As described here TypeScript introduces a foreach loop:

var someArray = [9, 2, 5];
for (var item of someArray) {
    console.log(item); // 9,2,5
}

But isn't there any index/key? I would expect something like:

for (var item, key of someArray) { ... }

Upvotes: 277

Views: 507727

Answers (5)

Arnavion
Arnavion

Reputation: 3968

See: Array.prototype.entries()

for (const [index, item] of someArray.entries()) { ... }

In TS this requires targeting ES2015 since it requires the runtime to support iterators, which ES5 runtimes don't. You can of course use something like Babel to make the output work on ES5 runtimes.

Upvotes: 98

David Sherret
David Sherret

Reputation: 106640

.forEach already has this ability:

const someArray = [9, 2, 5];
someArray.forEach((value, index) => {
    console.log(index); // 0, 1, 2
    console.log(value); // 9, 2, 5
});

But if you want the abilities of for...of, then you can map the array to the index and value:

for (const { index, value } of someArray.map((value, index) => ({ index, value }))) {
    console.log(index); // 0, 1, 2
    console.log(value); // 9, 2, 5
}

That's a little long, so it may help to put it in a reusable function:

function toEntries<T>(a: T[]) {
    return a.map((value, index) => [index, value] as const);
}

for (const [index, value] of toEntries(someArray)) {
    // ..etc..
}

Iterable Version

This will work when targeting ES3 or ES5 if you compile with the --downlevelIteration compiler option.

function* toEntries<T>(values: T[] | IterableIterator<T>) {
    let index = 0;
    for (const value of values) {
        yield [index, value] as const;
        index++;
    }
}

Array.prototype.entries() - ES6+

If you are able to target ES6+ environments then you can use the .entries() method as outlined in Arnavion's answer.

Upvotes: 475

Galdor
Galdor

Reputation: 1955

Or another old school solution:

var someArray = [9, 2, 5];
let i = 0;
for (var item of someArray) {
    console.log(item); // 9,2,5
    i++;
}

Upvotes: 12

Sylvain
Sylvain

Reputation: 1579

"Old school javascript" to the rescue (for those who aren't familiar/in love of functional programming)

for (let i = 0; i < someArray.length ; i++) {
  let item = someArray[i];
}

Upvotes: 71

Karanvir Kang
Karanvir Kang

Reputation: 2239

You can use the for..in TypeScript operator to access the index when dealing with collections.

var test = [7,8,9];
for (var i in test) {
   console.log(i + ': ' + test[i]);
}

Output:

 0: 7
 1: 8
 2: 9

See Demo

Upvotes: 21

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